There is growing evidence that the cell biologic and physiologic properties of renal ion transport systems are determined in part through their interactions with a variety of regulatory and structural proteins. We have demonstrated that renal ion transport polypeptides interact with tetraspan proteins, both in vitro and in situ. As their name implies, members of the tetraspan family are transmembrane proteins that are predicted to possess four membrane spanning segments. Genomic sequencing data suggest that mammals express about 30 tetraspan family members. All of these are characterized by short N and C terminal tails facing the cytoplasm and two relatively large extracellular loops in which are found a number of conserved residues that constitute a molecular signature present in most tetraspan family members. While much has been learned about the expression patterns of tetraspan proteins, less is known of their functions. It has been demonstrated that tetraspans form macromolecular complexes with a number of transmembrane proteins. These associations may be involved in regulating membrane protein distribution, stability and access to regulatory molecules. We find that interactions with tetraspans can exert tramatic effects on the subcellular localizations of renal transport proteins. Thus, interaction with the tetraspan protein CD63 induces the rapid endocytosis of the H,K-ATPase. In contrast, association with the tetraspan-like protein VIP17/MAL prevents the internalization of the aquaporin 2 water channel and increases its functional presence at the plasma membrane. We will examine the role that tetraspan proteins play in regulating the distribution and function of a number of important renal transport systems. Towards this end we will define the profile of tetraspan partners that associate with each of the individual members of a selected subset of renal ion transport proteins. We will assess the impact of these associations on the physiologic properties of these transport systems and we will define molecular domains of the tetraspan and transport proteins that participate in specific interactions. Finally, we will take advantage of knockout mouse models to measure the impact of tetraspan interactions on renal transport processes in situ.